Portable adjustable back support



FIG. 1 is a front perspective view of a portable adjustable back support showing my design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a bottom plan view thereof;

FIG. 4 is a front view thereof;

FIG. 5 is a rear view thereof. 

The ornamental design for a portable adjustable back support, as shown and described. 